Preloading by springs
By preloading bearings in small electric motors and similar applications it is possible to reduce operating noise. The bearing arrangement in this case comprises a single row deep groove ball bearing at each end of the shaft. The simplest method of applying preload is by a spring or spring "package" (fig 1). The spring acts on the outer ring of one of the two bearings; this outer ring must be able to be axially displaced. The preload force remains practically constant even when there is axial displacement of the bearing as a result of thermal expansion. The requisite preload force can be estimated from
F = k d
F = k d
|F||=||preload force, kN|
|k||=||a factor, see following|
|d||=||bearing bore diameter, mm|
Depending on the design of the electric motor, values of between 0,005 and 0,01 are used for the factor k. If preload is used primarily to protect the bearing from vibration damage when stationary, then greater preload is required and k = 0,02 should be used.
Spring loading is also a common method of applying preload to angular contact ball bearings used in high-speed grinding spindles. The method is not suitable, however, for bearing applications where a high degree of stiffness is required, where the direction of load changes, or where undefined shock loads can occur.